Currently, an absorbent body that is made of a water-absorbing agent that is made mainly from, for example, hydrophilic fiber such as pulp and acrylic acid (salt) is widely used in sanitary products such as disposable diapers, sanitary napkins, and incontinence pads so that bodily fluids are absorbed. In recent years, these sanitary products such as disposable diapers, sanitary napkins, and incontinence pads have been made higher in functionality and thinner, so that a larger amount of a water-absorbing agent tends to be used in a piece of sanitary product, and a water-absorbing agent tends to be contained in a larger amount with respect to an entire absorbent body that is made of, for example, a water-absorbing agent and hydrophilic fiber. Specifically, study has been carried out on a reduction in thickness of a sanitary product without a reduction in amount of water absorption by increasing a water-absorbing agent content in an absorbent body by (i) using a smaller amount of hydrophilic fiber having a small bulk specific gravity and (ii) using a larger amount of a water-absorbing agent having excellent water absorbency and a large bulk specific gravity.
Though such a sanitary product, in which a water-absorbing agent content is increased by decreasing a percentage of hydrophilic fiber, tends to be preferable from the viewpoint of simply storing a liquid, a problem rather arises in consideration of distribution and diffusion of a liquid in actual use of the sanitary product.
A water-absorbing agent that absorbs water turns to a soft gel-like water-absorbing agent. Thus, in a case where such a water-absorbing agent that is large in amount per unit volume absorbs water, a gel blocking phenomenon occurs. This phenomenon causes a considerable reduction in diffusibility of a liquid in a sanitary product. As a result, a part of the water-absorbing agent, which part is distant from a central area of the sanitary product and is therefore difficult for the liquid to reach, does not effectively function. This prevents an effect of increasing a water-absorbing agent content from being sufficiently exhibited, so that the sanitary product in actual use has absorbing ability that is much lower than a theoretical level.
In order that such a problem is avoided and absorbing ability of an absorbent body is maintained, a range of a ratio between hydrophilic fiber and a water-absorbing agent is inevitably restricted, so that thinning of sanitary products is also limited.
Examples of indices used to evaluate an improvement in gel blocking in a sanitary product encompass a fluid retention capacity under load (Absorbency Against Pressure (AAP) or Performance Under Pressure (PUP)), indicative of a water absorbent property under load, a saline flow conductivity (hereinafter abbreviated as “SFC”; see Patent Literature 1) and the like.
As a well-known technique for improving gel blocking, there have been known the following techniques: a technique in which crosslinking densities inside and outside of a water-absorbing agent are changed by a surface treatment; a technique for combining a surface treatment with (i) inorganic compound(s) as a liquid permeability improving agent, such as an inorganic microparticle and a polyvalent metal salt and/or (ii) cationic polymer compound(s) as a liquid permeability improving agent; a technique for improving water absorption performance, particularly liquid diffusibility; and a technique for controlling a reaction environment of a surface crosslinking treatment (see Patent Literatures 2 to 40). There have also been proposed various improvement techniques related to surface crosslinking (see Patent Literatures 72 to 74).
Meanwhile, not only liquid permeability but also water absorbing speed is also an important fundamental physical property of a water-absorbing agent. As a method for improving the water absorbing speed, there is known a technique for improving the water absorbing speed by increasing a specific surface area. Specific proposed examples of such a technique encompass a technique for minutely controlling a particle diameter, a technique for granulating fine powder having a large surface area, a technique for causing a hydrogel to be porous by freeze-drying, a technique for surface-crosslinking particles while granulating the particles, a technique for increasing a surface area of gel particles during gel grinding, foaming polymerization techniques such as introduction of microbubbles and addition of a surfactant, a technique for carrying out foaming and crosslinking after polymerization (see Patent Literatures 41 to 71), and the like.
An invention of a water-absorbing agent which simultaneously achieves the liquid permeability, the fluid retention capacity under load, and the water absorbing speed is necessary for development of sanitary products including an absorbent body that is thin and excellent in water absorbing speed. In general, the water absorbing speed and the specific surface area have a positive correlation, and the liquid permeability and the specific surface area have a negative correlation. This makes it extremely difficult to achieve both an increase in water absorbing speed that greatly depends on the surface area and an increase in liquid permeability and fluid retention capacity. Further, for improvement in water absorbing speed, there has been proposed a technique for making a water-absorbing agent porous (e.g., foaming polymerization). However, the water-absorbing agent that has been made porous has a smaller bulk specific gravity. Thus, the above technique not only causes an increase in transport cost and storage cost but also runs counter to thinning of sanitary products.
Further, society has recently been aging in advanced countries, and an elderly generation uses sanitary products such as disposable diapers at such a high rate that the elderly generation and infants are equally matched in usage rate of those sanitary products. Under the circumstances, a concentration of urine to be absorbed by sanitary products varies from a low concentration to a high concentration, and a water-absorbing agent is required to have water absorption performance called salt tolerance that is independent of salt concentration.